The present invention relates to controlling an actuator in a disc drive system. More particularly, the present invention relates to accurately determining the time of actuator unlatch in a disc drive.
In a disc drive system, it is sometimes desirable to restrain the actuator from movement. For example, transporting the disc drive or periods during which the disc drive is not in operation require that the actuator be restrained or "latched". An actuator has attached to its head-arm assembly, multiple magnetic read/write heads which are delicate instruments. Therefore, if the actuator is allowed to move when the drive sustains rough handling, such as during shipping, damage to the heads and/or the disc could occur.
The actuator may be restrained using a magnetic latch which holds the actuator in place over a specific area on the magnetic media disc. On zone bit recording drives, the initial demand current pulse which unlatches the actuator also accelerates the actuator. Immediately after unlatch, the actuator is moved to the outer zone of the disc to synchronize with the servo pattern of the disc. The initial acceleration demand current pulse is followed by an opposite deceleration pulse to slow the head to zero velocity to retrieve the servo pattern. Track seek operations begin after the servo pattern has been retrieved to move the actuator to specific disc locations.
To unlatch the actuator and accelerate it to a desired location on a magnetic media disc, a transconductance amplifier circuit converts input voltage, originating from the controlling code in the CPU, to demand current. The demand current is supplied to a voice coil motor (VCM) of the actuator. The initial demand current unlatches the actuator (in theory) and accelerates it to an outermost zone of the disc so that the servo pattern of the disc may be retrieved. After the actuator is unlatched and it begins to move, however, a "back" electromotive force (e.m.f.) voltage appears across the VCM. Thus, when the actuator is in motion, such as when it is enroute to the outermost zone of the disc, the transconductance amplifier circuit supplies voltage to the VCM to compensate for the back e.m.f. voltage.
The existing unlatch/track seek process is essentially an open loop procedure. As a result, there is no means for the controlling code to receive an indication that the actuator has actually been unlatched after the initial demand current pulse. If the first unlatch attempt fails, the entire process must be repeated with stronger and longer duration current pulses until the actuator is unlatched. Increasing the demand current, however, may cause the magnetic read/write head to catastrophically crash into the magnetic media disc. Damage to the magnetic head will occur if the latch is actually a weaker latch than anticipated and an excessive acceleration demand current has been applied.
Variations in manufacturing tolerances associated with the components of the latch (i.e., magnetic pole pieces, rubber housing etc.) have resulted in magnetic latches with poorly controlled latch forces. Present open-loop unlatch procedures often cannot unlatch strong latches on the first attempt. Poorly controlled latch forces have caused a significant problem with the conventional unlatch and acceleration process in that it is impossible to determine how much of the initial demand current was used to unlatch the actuator and how much resulted in acceleration. Thus, it is difficult to estimate the amount of deceleration current necessary to guarantee zero velocity at the end of acceleration.